Passive Optical Networks (PON) are point-to-multipoint optical fibre based access networks. Traditionally, one or more preliminary optical fibres extend from an optical line terminal (OLT) which is coupled to a core or backhaul network. The optical fibre or fibres terminate at a power splitter from which further optical fibres extend. These further or secondary optical fibres may in turn terminate at further power splitters with tertiary optical fibres extending from these splitters and so on. In this way, a number of subscriber equipment may be coupled to the OLT by this network of optical fibres and splitters. Because the primary fibre or fibres adjacent the OLT are common to all of the subscriber equipment, this bandwidth resource must be shared. Thus each subscriber equipment can only be allocated a small part of this shared bandwidth resource.
This natural limitation of the architecture of PONs limits the cost effectiveness of extending the fibre all the way to a subscriber's home. This is because the cost of installing a high bandwidth optical fibre to carry a small part of the shared bandwidth is too high. Therefore the fibre of a PON typically terminates at a street cabinet to be shared by a number of subscriber equipment. The street cabinet houses an optical network terminal (ONT). The last few meters or kilometer from the street cabinet to a subscribers home is typically bridged using existing twisted pair copper cables or radio links. The bandwidth resources of the fibre to the street cabinet are typically shared amongst a group of subscribers using an Ethernet switch. Whilst the downstream channels of a PON are broadcast to each ONT, an upstream channel is provided for each ONT on the PON using a time division multiple access (TDMA) protocol.
The use of 10 Gb/s transmission in PON is under study to better exploit the potential fibre capacity and provide greater bandwidth to each subscriber equipment. Similarly, the use of wave division multiplexing (WDM) techniques is also under study for the same purpose. However, the challenge in access networks is to maintain a low overall cost. This enhanced capacity may enhance the extent of fibre penetration when considering the implementation, extension and/or optimisation of an existing PON. For example the fibre may extend to a street cabinet, a kerb outside a subscribers home, or directly into the subscribers home itself.